Method of Induction Hardening

ABSTRACT

Long work of Fe based alloy while being rotated at a given rotation speed under the action of rotating chucks is heated by high-frequency heating coil to thereby accomplish the heating step. In the first cooling step after termination of the heating, the long work while having its rotation speed increased is brought into contact with first to fourth correction rollers, and in that state is cooled so as to fall within a temperature range of Pf temperature or below to over Ms temperature. The time spent in the cooling preferably ranges from 5 to 10 sec. In the subsequent second cooling step, the rotation speed of the long work is reduced, preferably so as to be equal to that at the heating, and cooling of the long work is continued.

TECHNICAL FIELD

The present invention relates to an induction hardening method, more particularly to an induction hardening method for a long work of an Fe-based alloy extending along the axis line.

BACKGROUND ART

Induction hardening treatment methods for a rod-shaped long work, such as a drive shaft for an automobile engine, include single-shot hardening methods of hardening the entire long work at once, and transfer hardening methods containing placing a high-frequency heating coil around a part of the long work, and transporting the long work in the axial direction to sequentially heat the parts of the long work. In the hardening methods, the long work is heated by a high-frequency heating coil, and then cooled by a cooling liquid.

In the single-shot hardening methods, the long work is clamped at both ends, and forced to be rotated (for example, see Patent Document 1). The long work is heated in this state, then cooled while being rotated, and thereby hardened. A correction roller is placed in the vicinity of the long work. When the long work is distorted in the cooling, the distorted portion of the long work rotated is brought into contact with the correction roller to correct the distortion.

Patent Document 1: Japanese Laid-Open Patent Publication No. 04-141523 DISCLOSURE OF THE INVENTION

However, the distortion cannot be sufficiently eliminated only by the correction roller, and generally a cold distortion eliminating process is carried out after the hardening treatment. The long work is often cracked by the distortion eliminating process, and a magnetic crack detection is carried out to detect the cracking. Thus, the number of processes is increased, so that it is difficult to improve the production efficiency of the long work obviously.

A general object of the present invention is to provide an induction hardening method capable of preventing the distortion of a long work.

A principal object of the present invention is to provide an induction hardening method not requiring distortion elimination and magnetic crack detection processes.

Another object of the present invention is to provide an induction hardening method capable of producing a long work having an approximately uniform metal structure over the entire work.

A further object of the present invention is to provide such a simple induction hardening method that the distortion of a long work can be corrected even when a correction roller is held for some reasons.

A still further object of the present invention is to provide an induction hardening method having an improved production efficiency of a long work.

According to an aspect of the present invention, there is provided an induction hardening method for hardening a long work containing an Fe-based alloy, wherein the long work is heated by high-frequency induction heating while being rotated, and when the long work is distorted in the hardening, the long work is brought into contact with a correction roller to correct the distortion.

The induction hardening method comprises:

a heating step of heating the long work by the high-frequency induction heating to a first temperature range, within which an austenite is formed, while rotating the long work at a first rotational speed;

a first cooling step of cooling the long work to a second temperature range of higher than the martensite start temperature and at most the pearlite finish temperature while rotating the long work at a second rotational speed higher than the first rotational speed; and

a second cooling step of further cooling the long work having a temperature within the second temperature range while rotating the long work at a third rotational speed lower than the second rotational speed.

The Fe-based alloy used as a material of the long work has a structure, from which the distortion can be easily eliminated, at a temperature lower than the pearlite finish temperature (the Pf temperature). Thus, when the long work is distorted in the cooling step at a maximum rotational speed after the heating step, the distortion can be efficiently eliminated by bringing the long work into contact with the correction roller. This is because the correction roller and the long work can be brought into contact at a high contact frequency.

Thus, in the present invention, the distortion is hardly present in the resultant long work after the second cooling step. Therefore, the cold distortion eliminating process can be omitted, and naturally also the magnetic crack detection for detecting cracks in the long work due to the cold distortion eliminating process can be omitted. The long work production efficiency can be improved by omitting the processes.

In addition, the resultant long work has an approximately uniform metal structure and properties over the entire long work.

The first cooling step is carried out at the maximum rotational speed until the long work is cooled to the predetermined temperature range (the second temperature range). Specifically, the second temperature range is a range of higher than the martensite start temperature (the Ms temperature) and at most the Pf temperature. It is particularly preferred that the long work is cooled to a temperature immediately above the Ms temperature in the first cooling step.

When the rotational speed of the correction roller is changed, practically the speed is gradually increased or decreased due to the inertia. Thus, the rotational speed is in the middle of the increase immediately after the start of the first cooling step, and the rotational speed is in the middle of the decrease immediately after the start of the second cooling step.

It is preferred that at least one correction roller can be freely rotated independently from other correction rollers. In this case, the distortion can be corrected with simple structure, even when the other correction rollers are fixed for some reasons.

In the first cooling step, the cooling time is selected depending on the size, weight, and hardness of the long work. For example, in a case where the long work has a cylindrical shape, as the diameter is increased, the cooling time is lengthened.

The rotational speeds in the heating step and the second cooling step (i.e. the first rotational speed and the third rotational speed) may be equal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing a hardening apparatus for an induction hardening method according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view along the line II-II of FIG. 1;

FIG. 3 is a top plan view showing the principal part of the hardening apparatus of FIG. 1;

FIG. 4 is a CCT curve of an S40CM material; and

FIG. 5 is a graph showing a rotational speed in each step of the induction hardening method.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the induction hardening method of the present invention will be described in detail below with reference to accompanying drawings.

The induction hardening method of this embodiment contains a heating step, a first cooling step, and a second cooling step. Thus, a long work is heated in the heating step, and is cooled in the first cooling step and the subsequent second cooling step.

FIG. 1 is a side view of a hardening apparatus 10 for carrying out the heating step, the first cooling step, and the second cooling step, FIG. 2 is a cross-sectional view along the line II-II of FIG. 1, and FIG. 3 is a top plan view of the principal part. The hardening apparatus 10 has a correction mechanism 12, rotatable chucks 14 a, 14 b for forming a clamp mechanism, a high-frequency heating coil 16, and a movable cooling jacket (not shown).

The correction mechanism 12 has a base 18 and first to fourth bearings 20 a to 20 d formed thereon. A first rotation shaft 22 a is supported by the first and second bearings 20 a and 20 b, while a second rotation shaft 22 b is supported by the third and fourth bearings 20 c and 20 d. Of course the first and second rotation shafts 22 a and 22 b are rotatable independently.

As shown in FIG. 1, first and second correction rollers 24 a and 24 b are positioned and fixed onto the first rotation shaft 22 a, and third and fourth correction rollers 24 c and 24 d are positioned and fixed onto the second rotation shaft 22 b, such that the third and fourth correction rollers 24 c and 24 d do not interfere the first and second correction rollers 24 a and 24 b. The peripheral walls of the first to fourth correction rollers 24 a to 24 d are positioned at a predetermined distance from the peripheral wall of a long work LW.

The rotatable chucks 14 a, 14 b of the clamp mechanism can be moved close to and away from the ends of the long work LW, and in other words can be opened and closed. When the rotatable chucks 14 a, 14 b are closed, the ends of the long work LW are pressed by the rotatable chucks 14 a, 14 b, whereby the long work LW is clamped.

The rotatable chucks 14 a, 14 b can be rotated at a controlled rotational speed under the action of a rotation controlling motor (not shown). The rotational speed can be controlled by changing a rotational force from the rotation controlling motor.

The high-frequency heating coil 16 comprises arches 26 a, 26 b which are located near the ends of the long work LW, and are curved along the upper half of the long work LW, and straight portions 28 a, 28 b which are formed to link the ends of the arches 26 a, 26 b. Further, arms 30 a, 30 b are disposed on the arches 26 a, 26 b respectively, and one end of each arm 30 a, 30 b is supported by an elevating mechanism (not shown). When the arms 30 a, 30 b are moved downward or upward by the elevating mechanism, the high-frequency heating coil 16 is moved close to the long work LW to surround the upper half or moved away from the upper half.

The long work LW is not particularly limited as long as the height (the length in the axial direction) is equal to or more than the bottom diameter, width, and depth. Preferred examples of the long works LW include drive shafts.

The induction hardening method of this embodiment is carried out as follows.

First, the rotatable chucks 14 a, 14 b are closed to clamp the ends of the long work LW such as a drive shaft. Then, the arms 30 a, 30 b of the high-frequency heating coil 16 are moved downward by the elevating mechanism, and finally the upper half of the long work LW is surrounded by the high-frequency heating coil 16 as shown in FIG. 2.

The rotatable chucks 14 a, 14 b are rotated by the rotation controlling motor, and thus the long work LW is rotated. For example, the rotational speed may be 100 to 200 rpm.

In this state, the high-frequency heating coil 16 is energized to start the heating step, so that the long work LW is heated to about 900° C. to 950° C. by electromagnetic induction heating. Thus, the heating step of the induction hardening treatment is started. In the electromagnetic induction heating, austenite transformation is caused in the metal structure of the long work LW composed of an Fe-based alloy.

After a predetermined time, the energization of the high-frequency heating coil 16 is stopped, the heating jacket is moved away from the long work LW, and the rotational speed of the rotatable chucks 14 a, 14 b are increased. For example, the rotational speed of the rotatable chucks 14 a, 14 b may be finally increased to 240 to 300 rpm.

Immediately after the heating jacket is moved away from the long work LW, the long work LW is surrounded by the movable cooling jacket.

The movable cooling jacket has a semi-cylindrical shape, and is moved in the longitudinal direction of the long work LW while surrounding a part of the upper half of the long work LW. An injector for spraying a cooling liquid onto the long work LW is disposed on the inner periphery wall of the movable cooling jacket.

Thus, the long work LW is cooled by the cooling liquid emitted from the inner periphery wall of the movable cooling jacket, so that the first cooling step is started. The movable cooling jacket is moved in the longitudinal direction of the long work LW, whereby the entire long work LW is cooled.

In this cooling step, a ferrite or a pearlite is formed in the metal structure of the long work LW (the Fe-based alloy). The metal structure of the long work LW is changed by the ferrite and pearlite formation, and a part of the long work LW may be swelled to generate a distortion before the long work LW is cooled to the pearlite finish temperature (the Pf temperature), at which the pearlite formation is finished. In this case, the distorted part is brought into contact with one of the first to fourth correction rollers 24 a to 24 d at a rate of 0.83 to 5 times/minute, so that the distortion of the long work LW is corrected. Of course the cooling liquid is continuously emitted from the movable cooling jacket during this correction.

In the temperature range between the austenite forming temperature and the Pf temperature, the long work LW has a structure from which the distortion can be easily eliminated. Therefore, in the first cooling step, the distortion of the long work LW can be efficiently eliminated by bringing the long work LW into contact with the first to fourth correction rollers 24 a to 24 d while rotating the long work LW at the maximum rotational speed.

In this embodiment, the rotational speed of the long work LW in the first cooling step is higher than that in the high-frequency heating step, so that the first to fourth correction rollers 24 a to 24 d and the long work LW can be brought into contact at a high contact frequency. As a result, the effect of correcting the distortion of the long work LW is improved.

Further, in this embodiment, the resultant long work LW has an approximately uniform metal structure, and thereby has uniform properties, over the entire work.

The above rotational speed of the rotatable chucks 14 a, 14 b is maintained (i.e., the first cooling step is continued) until the long work LW is cooled to a predetermined temperature range, within which a large distortion is hardly generated in the long work LW, specifically to the Pf temperature or less. When the long work LW is cooled to a temperature lower than the martensite start temperature (the Ms temperature) in the first cooling step, a so-called hardening crack may be caused by a martensite formed. In view of this problem, in the first cooling step, the long work LW is cooled to a temperature higher than the Ms temperature.

In short, at the end of the first cooling step, the temperature of the long work LW is equal to or lower than the Pf temperature and higher than the Ms temperature. It is preferred that the long work LW is cooled to a temperature immediately above the Ms temperature in the first cooling step. In this case, the dimensional accuracy of the long work LW is improved.

The Pf and Ms temperatures are obtained from a continuous cooling transformation curve (CCT curve) before the first cooling step. For example, in the case of using an S40CM as a material of the long work LW, the Pf and Ms temperatures thereof can be obtained from a CCT curve shown in FIG. 4. In FIG. 4, Fs represents a ferrite start temperature at which ferrite formation is started, and Ps represents a pearlite start temperature at which pearlite formation is started.

In the first cooling step, when the cooling time is excessively short, the long work LW still has a high temperature and is returned to the heated state, resulting in a lowered hardness. On the other hand, when the cooling time is excessively long, the treatment efficiency is deteriorated. Thus, in the first cooling step, the cooling time is controlled such that the hardness of the long work LW is not lowered, and the treatment efficiency is not deteriorated.

The cooling time is selected depending on the diameter, weight, and hardness of the long work LW. Thus, the cooling time is not determined depending on one factor. For example, when the long work LW is composed of an S40CM and has a cylindrical shape with a diameter of about 20 cm, the cooling time may be 10 to 20 seconds.

After the first cooling step, the long work LW is further cooled by spraying the cooling liquid from the movable cooling jacket while reducing the rotational speed of the rotatable chucks 14 a, 14 b (and the long work LW) in the second cooling step. In the second cooling step, the cooling liquid spraying may be stopped, and the temperature of the cooling liquid may be lower than that in the first cooling step.

For example, the rotational speed in the second cooling step may be equal to that in the heating step. Thus, for example, the rotational speed may be 100 or 150 rpm in both the heating step and the second cooling step. It is preferred that the rotational speed is 180 rpm in the heating step and the second cooling step. In this case, the long work LW can be prevented from being deformed due to the martensitic transformation.

After a predetermined cooling time, the entire induction hardening treatment is completed. The relation between the time and the rotational speed in the above steps is shown in FIG. 5. In the example of FIG. 5, the rotational speed is 180 rpm in the heating step and the second cooling step, and 250 rpm in the first cooling step.

The resultant long work LW has little or no distortion, and does not have to be subjected to a cold distortion eliminating process for removing the distortion. Also a magnetic crack detection process for detecting cracks in the long work LW due to the cold distortion eliminating process is not required naturally. Thus, the long work LW treatment efficiency, and the long work LW production efficiency are improved.

Though the S40CM material is used in the above embodiment, the material of the long work LW is not particularly limited as long as it is an Fe-based alloy. The Pf and Ms temperatures, etc. of the material other than the S40CM can be obtained using the corresponding CCT curve.

Further, the shape of the long work LW is not limited to the above cylindrical shape with the spherical bottom surfaces. The long work LW may have a polygonal column shape with polygonal bottom surfaces. The bottom surfaces may have different shapes. 

1. An induction hardening method for hardening a long work containing an Fe-based alloy, wherein said long work is heated by high-frequency induction heating while being rotated, and when said long work is distorted in the hardening, a correction roller is brought into contact with said long work to correct the distortion, said method comprising: a heating step of heating said long work by said high-frequency induction heating to a first temperature range, within which an austenite is formed, while rotating said long work at a first rotational speed; a first cooling step of cooling said long work to a second temperature range of higher than the martensite start temperature and at most the pearlite finish temperature while rotating said long work at a second rotational speed higher than said first rotational speed; and a second cooling step of further cooling said long work having a temperature within said second temperature range while rotating said long work at a third rotational speed lower than said second rotational speed.
 2. An induction hardening method according to claim 1, wherein said first rotational speed is equal to said third rotational speed.
 3. An induction hardening method according to claim 1, wherein said long work is a drive shaft.
 4. An induction hardening method according to claim 1, wherein said heating step, said first cooling step, and said second cooling step are carried out using a plurality of correction rollers and at least two rotation shafts for axially supporting at least one of said correction rollers, said rotation shafts being rotatable independently from each other. 